Wireless local area networks (WLANs) are gaining in popularity, and new wireless applications are being developed. The original WLAN standards, such as “Bluetooth” and IEEE 802.11, were designed to enable communications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEE working groups have defined the 802.11a, 802.11b and 802.11g extensions to the original standard, in order to enable higher data rates. The 802.11a standard, for example, envisions data rates up to 54 Mbps over short distances in a 5 GHz band, while 802.11b defines data rates up to 22 Mbps in the 2.4 GHz band. In the context of the present patent application and in the claims, the term “802.11” is used to refer collectively to the original IEEE 802.11 standard and all its variants and extensions, unless specifically noted otherwise.
The theoretical capability of new WLAN technologies to offer high communication bandwidth to mobile users is severely hampered by the practical limitations of wireless communications. Indoor propagation of radio frequencies is not isotropic, because radio waves are influenced by building layout and furnishings. Therefore, even when wireless access points are carefully positioned throughout a building, some “black holes” generally remain—areas with little or no radio reception. Furthermore, 802.11 wireless links can operate at full speed only under conditions of high signal/noise ratio. Signal strength scales inversely with the distance of the mobile station from its access point, and therefore so does communication speed. A single mobile station with poor reception due to distance or radio propagation problems can slow down WLAN access for all other users in its basic service set (BSS—the group of mobile stations communicating with the same access point in a conventional 802.11 WLAN).
The natural response to these practical difficulties would be to distribute a greater number of access points within the area to be served. If a receiver receives signals simultaneously from two sources of similar strength on the same frequency channel, however, it is generally unable to decipher either signal. The 802.11 standard provides a mechanism for collision avoidance based on clear channel assessment (CCA), which requires a station to refrain from transmitting when it senses other transmissions on its frequency channel. In practice, this mechanism is of limited utility and can place a heavy burden on different BSSs operating on the same frequency channel.
Therefore, in high data-rate 802.11 WLANs known in the art, access points in mutual proximity must use different frequency channels. Theoretically, the 802.11b and 802.11g standards define 14 frequency channels in the 2.4 GHz band, spaced about 5 MHz apart. The usable 802.11b signal in each channel, however, occupies approximately 20-25 MHz of the frequency spectrum. For this reason, and because of regulatory limitations, 802.11 WLANs operating in the 2.4 GHz band in the United States actually have only three frequency channels from which to choose (channels 1, 6 and 11, with channel spacing of 25 MHz. In the 5 GHz band, with channel spacing of 20 MHz, a larger number of frequency channels is available, but the choice is still limited.) Access points are typically set to operate on one of these channels, and mobile stations tune their radios to the available frequency.
The actual data rate for communication between an access point and mobile stations that it serves is chosen from a list of possible data rates by negotiation between the access point and mobile stations. For example, “multirate support” in the 802.11 environment is described in section 9.6 (page 95) of ANSI/IEEE Standard 802.11 (1999 Edition), entitled Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, which is incorporated herein by reference. As a rule, maximum-rate communication is possible only when the access point and mobile stations are close together and subject to good signal/noise conditions. The data rate typically drops as the distance between the access point and mobile stations increases. Each access point and mobile station announces the rates that may be used to communicate with it in the “supported rates” field of management frames that it transmits, as described in section 7.2.3 of the above-mentioned standard.